The present disclosure relates to the field of radiography and, in particular, relates to computed tomography scanners. Even more particularly, the present disclosure relates to an adjustable x-ray beam collimator for use with a computed tomography scanner.
In computed tomography a patient to be examined is positioned in a scan circle of a computed tomography (CT) scanner. A shaped x-ray beam is then projected from an x-ray source through the scan circle and the patient, to an array of radiation detectors. By rotating the x-ray source and the detector array about the patient (about a z-axis of the scanner), radiation is projected through an imaged slice of the patient to the detectors from a multiplicity of directions. From data provided by the detectors, an image of the scanned slice of the patient is constructed.
Within the x-ray source, an electron beam strikes a focal spot or line on an anode, and x-rays are generated at the focal spot and emitted along diverging linear paths in an x-ray beam. Collimators are normally employed for shaping a cross-section of the x-ray beam, and for directing the shaped beam toward the detector array. A pre-patient collimator is positioned between the x-ray source and the patient, while a post-patient collimator is positioned between the patient and the detector array. Conventional collimators generally comprise a plate-like structure provided with a rectangular slit of uniform width for producing a rectangular beam cross-section, as desired with systems employing a rectangular detector array.
Some CT scanners allow for flexibility in the number and thickness of slices acquired during a scan. In such scanners, the x-ray beam is collimated such that its cross-section irradiates a particular row, or rows of detectors, without irradiating adjacent rows of detectors not utilized for that scan.
What is desired, therefore, is an improved collimator for shaping a cross-section of a beam of energy, and for directing the shaped beam toward a desired target, such as a detector array. What is further desired is an adjustable collimator that produces beam cross-sections of variable widths. Preferably, the collimator will also produce beam cross-sections of variable, yet uniform widths, such that cross-sections will irradiate a specific row, or rows of detectors, without irradiating adjacent rows of detectors.
The present disclosure, accordingly, is director to an adjustable collimator for collimating a beam of energy emitted from a focal spot of a beam source. The collimator is particularly intended for collimating an x-ray beam of a computed tomography scanner after the x-ray beam has passed through a patient being scanned.
The collimator generally includes two elongated parallel plates arranged side by side to define a collimating slit between the plates. At least one of the plates is movably relative to the other plate for varying a width of the collimating slit. The collimator also includes a movable cam operatively arranged with respect to the at least one movable plate such that movement of the cam in a first direction causes the width of the collimating slit to increase, while movement of the cam in a second direction causes the width of the collimating slit to decrease.
According to one aspect of the disclosure, the cam is rotatably movable and the collimator includes a motor having has a rotatable shaft coupled to the cam for controlling the width of the collimating slit. According to another aspect, the motor comprises a step motor for providing the collimating slit with predefined, discrete widths. Preferably, the variable slit of the collimator has a uniform width and is curved about a common axis of curvature for alignment with a focal spot of a beam source, such that the collimating slit produces beam cross-sections of variable, yet uniform widths.
The present disclosure also provides a computed tomography scanner including a collimator as described above, and further including an annular gantry rotatable about a rotation axis, a beam source mounted within the gantry and having a focal spot for emitting an x-ray beam through the rotation axis, and an array of x-ray detectors for receiving the x-ray beam from the focal spot. The collimator is mounted within the gantry between the focal spot and the detectors for collimating the x-ray beam. According to one aspect, the collimator is positioned between the axis of rotation and the detector array to act as a xe2x80x9cpost-patientxe2x80x9d collimator.